2-10
Functional Description
You must give special consideration to the direct measurement of
low-level signals with the DAS-1801ST board. When using the ±20mV, 0
to 20mV, ±100mV, or 0 to 100mV ranges, measurement throughput drops
for two reasons:
●
The amplifier cannot settle quickly enough (particularly the ±20mV
and 0 to 20mV ranges).
●
Noise in the measurements is higher and thus can require
post-acquisition filtering (averaging) to achieve accurate results.
The DAS-1801ST has best noise performance if presented with a perfect
signal in these ranges, but perfect signals are virtually non-existent in the
real world. Since the DAS-1801ST has a very high bandwidth (bandwidth
for low-level signals is about 8 to 10MHz), any noise is amplified and
digitized. As a result, you must carry out the measurement of low-level
signals carefully to minimize noise effects.
Low-level transducers are best used with signal conditioning. Use the
±20mV, 0 to 20mV, ±100mV, and 0 to 100mV ranges with the differential
input mode.
The following tables show throughput for various configurations. Note
that these throughputs are based on driving the input with an ideal voltage
source. The output impedance and drive of the source are far more critical
when making large gain changes between two channels whose inputs are
at opposite extremes of their input ranges, as when a signal near
−
20mV
is measured after a signal at near +5V. You will get better performance
driving adjacent channels at the same gain. The source needs to be able to
drive both the capacitance of the cable and the RC (resistor-capacitor)
product of the multiplexer resistance and the output capacitance of the
multiplexer and board. The multiplexer is typically about 360
Ω
(1k
Ω
maximum) in series with 90pF output capacitance.
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